When a temperature difference is generated between both ends of a thermoelectric conversion material, an electromotive force (a thermal electromotive force) is generated in proportion to the temperature difference. The phenomenon that thermal energy is converted into electrical energy in a thermoelectric conversion material is known as the Seebeck effect. The electromotive force V that is generated is expressed as V=SΔT, where ΔT is a temperature difference and S is the Seebeck coefficient peculiar to the material.
In a thermoelectric conversion material that exhibits isotropic physical properties, the electromotive force generated by the Seebeck effect is generated only in the direction in which the temperature difference has been generated. On the other hand, due to the inclined arrangement of the crystal axes, the thermoelectric conversion material that exhibits anisotropy in its electrical transport properties generates an electromotive force in the direction orthogonal to the direction in which the temperature difference has been generated. The electrical transport properties denote the behavior of electrons and positive holes having electric charges that move in a substance. As described above, the phenomenon that due to the inclined arrangement of the crystal axes of the material, an electromotive force is generated in the direction that is different from the direction in which the temperature difference has been generated (a heat flow direction) is referred to as an anisotropic thermoelectric effect or an off-diagonal thermoelectric effect.
FIG. 13 is a diagram of a coordinate system for explaining the anisotropic thermoelectric effect. As shown in FIG. 13, the crystal axes abc of the sample 101 are inclined to the spatial axes xyz. In the sample 101, when a temperature difference ΔTz is applied in the direction along the z axis, an electromotive force Vx is generated in the direction orthogonal to the z axis, i.e. the direction along the x axis. The electromotive force Vx is represented by Formula (1):
                              [                      Mathematical            ⁢                                                  ⁢            Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                V          x                =                              l                          2              ⁢              d                                ⁢          Δ          ⁢                                          ⁢                                    T              z                        ·            Δ                    ⁢                                          ⁢                      S            ·            sin                    ⁢                                          ⁢          2          ⁢          α                                    (        1        )            where 1 denotes the width of the sample 101, d denotes the thickness of the sample 101, α denotes the inclination angle of the a-b plane to the surface (the x-y plane) of the sample 101, and ΔS denotes the difference (the difference that occurs due to anisotropy) between the Seebeck coefficient Sc in the c-axis direction and the Seebeck coefficient Sab in the a-b in-plane direction.
Conventionally, a radiation detector using an inclined layered thin film of YBa2Cu3O7-d (hereinafter referred to as “YBCO”) has been proposed as a radiation detector that utilizes the anisotropic thermoelectric effect (see, for example, Patent Literature 1). The inclined layered thin film denotes a thin film that is layered on a substrate and that has a structure in which the crystal axis is inclined to the surface of the substrate and a plurality of layers are layered together. The YBCO thin film has an anisotropic crystal structure in which a CuO2 layer having electrical conductivity and Y and BaO layers that have insulation properties are layered alternately along the c-axis direction. When the YBCO thin film is layered (inclined and layered) on a suitable substrate surface in such a manner that the c axis is inclined to the substrate surface, a similar system to that shown in FIG. 13 is formed. The CuO2 plane corresponds to the a-b plane shown in FIG. 13. When an electromagnetic wave is incident on the surface of the YBCO thin film that has been inclined and layered as described above, a temperature difference is generated in the direction perpendicular to the surface of the YBCO thin film. As a result, an electromotive force is generated in the direction parallel to the surface of the YBCO thin film by the anisotropic thermoelectric effect. Reading this electromotive force allows the electromagnetic wave that has been incident on the surface of the YBCO thin film to be detected. A radiation detector using the YBCO thin film can detect an electromagnetic wave at a sensitivity of approximately 100 mV/K.
From Formula (1), the electromotive force Vx that is generated by the anisotropic thermoelectric effect is proportional to the difference ΔS that occurs due to anisotropy of the Seebeck coefficient, the aspect ratio 1/d of a sample, and a sine value of sin 2α of an angle that is twice the inclination angle α. In the YBCO thin film, the difference ΔS is smaller than 10 μV/K, and the upper limit for maintaining the inclination angle α of the CuO2 plane at a single angle is limited to approximately 10 to 20° (see, for example, Non-Patent Literature 1 and Non-Patent Literature 2). Accordingly, the radiation detector that includes the YBCO thin film used therein cannot be said to have sufficiently high sensitivity for being used practically. In order to improve the sensitivity of a radiation detector that includes an inclined layered thin film used therein, there are methods in which, for example, a material with a larger difference ΔS is used and the inclination angle α of the thin film is brought close to 45 degrees as much as possible. Since the range of the inclination angle α in the inclined layered thin film depends on the combination of the thin film material and the substrate material on which the thin film material is layered, it is preferable that a suitable substrate material be selected so that the inclination angle α can be controlled widely up to around 45°.
Patent Literature 1 discloses a radiation detector in which a YBCO thin film partially doped with Pr is used. According to Patent Literature 1, the radiation detector has a sensitivity approximately twenty times higher than that of a radiation detector with a non-doped YBCO thin film used therein. It is suggested that the reason for this is because the Seebeck coefficient of the YBCO thin film is increased by Pr doping. However, Non-Patent Literature 3 describes that in a Pr-doped YBCO thin film, the Seebeck coefficient increases in the direction of the a-b in-plane but remains unchanged in the c-axis direction. Furthermore, Non-Patent Literature 3 describes that the difference ΔS becomes smaller in the Pr doping range that is employed in the YBCO thin film used for the radiation detector of Patent Literature 1. Therefore, using light with a wavelength (308 nm) that was different from light with a wavelength of 248 nm used in Patent Literature 1, the response of the Pr-doped YBCO thin film to light irradiation was actually measured by the same experimental method as in Non-Patent Literature 3. As a result, it was confirmed that the Pr-doped YBCO thin film had a smaller electromotive force that was generated by the anisotropic thermoelectric effect.
The improvement in sensitivity of the radiation detector with the Pr-doped YBCO thin film used therein is probably attributed to an increase in absorption coefficient of the YBCO thin film with respect to light with a wavelength of 248 nm due to Pr doping. That is, although the radiation detector of Patent Literature 1 is highly sensitive to light with a wavelength of 248 nm, it cannot be said that the detection sensitivity is improved in other wavelength ranges.